I too suffer from the rattling passenger seat when no one is sitting next to me. I took it to the dealer and I drove him around while he was sitting in the back. He noticed it and put in a request to Munich concerning this problem. After a week I popped back in and he told me this problem seems to be hitting a lot of X6s and they think it is a manufacturing issue of the comfort seats. He offered to replace the whole seat for me which I haven't done yet since he said if it is manufacturing then until it gets solved the new will probably do the same.

I'm waiting to see if this will become a big enough issue for BMW to address.

D

I have the same rattle without comfort seats. I believe it is the seat bottom sensor that detects a passenger for airbag enabling.

If I pressed down firmly on the center of the seat and let go, the rattle goes away. So I just do this if it starts to rattle.

Replacing your seat won't make any difference and might even add more problems.

Finally after taking my X6 in several times, they found a solution for a large part of the rattling from the passenger seat. Here are the repair notes from BMW:
CUSTOMER STATES THERE IS A SQUEAK/RATTLECOMING FROM VEHICLE. CUSTOMER PERFORMED RIDE ALONG WITH SHOP FOREMAN (with no rider in passenger seat). NOISE COMING FROM SEAT BOTTOM WHEN LEFT SIDE OF SEAT BOLSTER ON PASSENGER SIDE IS ADJUSTED.

BMW-GT, that's great. I've had two dealerships look at it and they couldn't/wouldn't find the problem--still rattles. I might poke around there myself and see what I can do. Though I now also have a dash rattle from between the iDrive screen and the glove box... I think bad city roads + X6 stiff suspension = too much for all these plastic parts.